Alessandra Geddo Lehmann

Rietveld refinements of the paraelectric and ferroelectric structures of PbFe0.5Ta0.5O3

The aristotype and the two ettotypes of the complex perovskite PbFe0.5Nb0.5O3 have been refined by the Rietveld method from neutron and x-ray powder diffraction data. To avoid errors due to sample off-stoichiometry or inhomogeneity, only powders obtained by grinding single crystals were analysed. At 523 K the cubic phase (space group Pmm), which is known to be stable for T > 376 K, has the lattice parameter ac = 4.010(1) A and is characterized by disorder of the lead and oxygen atoms, as in most Pb-based complex perovskites. At 363 K the intermediate phase, stable for 355 K < T < 376 K, is confirmed to be tetragonal, space group P4mm, with lattice parameters at = 4.007(1) A and ct = 4.013(1) A. The best refinement for the low-temperature structure (T < 355 K) is obtained, both at 250 and 80 K, in monoclinic symmetry, space group Cm. The monoclinic cell at 250 K has parameters am = 5.674(1) A, bm = 5.663(1) A, cm = 4.013(1) A and = 89.84°(2).

Ferroelastic symmetry changes in the perovskite PbFeY0.5Ta0.5O3

The ferroelastic symmetry changes of the lead-based complex perovskite PbFeY0.5Ta0.5O3 are investigated by means of single crystal x-ray diffraction and observations in polarized light microscopy at different temperatures. The position of the 400 and 222 cubic reflections is followed between 80 K and 300 K. Results indicate the presence of two structural transitions at about 270 K and 220 K. Such temperatures define the stability range of three different phases. The high-temperature phase (T > 270 K) is the optically anomalous paraelectric prototype, the optical symmetry of which is uniaxial (tetragonal) in spite of the diffractometric (pseudo)cubic symmetry. The intermediate derived phase (270 K > T > 220 K) is tetragonal. This tetragonal phase coexists with a monoclinic one for 220 K > T > 200 K. Below 200 K only the monoclinic phase is stable. The (pseudo)cubic-to-tetragonal transition seems to be of the second order. The tetragonal-to-monoclinic one is of the first order.

The disordered structure of the complex perovskite

The structure of the complex perovskite PbFe0.5Ta0.5O3 (PFT), which belongs, according to the literature, to the ferroic species m3mF3m, was refined by single-crystal and powder x-ray diffraction at room temp., i.e. within the range of stability of the cubic phase. Attention was paid to the possible presence of tetragonal spontaneous strain, suggested by the anomalous uniaxial birefringence of each (100)cub pyramidal growth sector. A crystal cut from a single growth sector was measured and compared with an as-grown one. Several models, both cubic (Pm3m) and tetragonal (P4/mmm), these latter used in refining the single growth sector, are compared. At. coordinates are given. The main feature of all the refined structures is the positional disorder at the Pb site, the importance of which in connection with the ferroic transitions is briefly discussed.

Structural characterization of Lu{sub 1.8}Y{sub 0.2}SiO{sub 5} crystals

The structural and vibrational properties of Lu{sub 1.8}Y{sub 0.2}SiO{sub 5} (LYSO) single crystals were investigated by means of Raman spectroscopy and x-ray diffraction measurements. Unit cell parameters and bond lengths were determined by Rietveld refinement of the collected x-ray diffraction powder spectra. By comparison with the vibrational spectra of the parent compounds Lu{sub 2}SiO{sub 5} and Y{sub 2}SiO{sub 5} and by using polarized Raman measurements, we propose the assignment of the principal vibrational modes of LYSO crystals. The strict connection of Raman spectra of the LYSO solid solution and of the pure lutetium and yttrium crystals, as well as the analysis of the polarized measurements, confirms that LYSO structure adopts the C2/c space group symmetry. The structural analogies of LYSO with the pure compound Lu{sub 2}SiO{sub 5} are further shown by means of high pressure Raman spectroscopy, and the possibility of considering the LYSO crystal analogous to the LSO structure with a tensile stress between 0.25 and 0.80 GPa is discussed.

Investigation of a Peaked Feature in the Magnetic Susceptibility of YBa2Cu3O6.30 Samples

The static magnetic susceptibility of two different YBa2Cu3O6.30 samples has been measured at different fields in the range 2 - 300 K, using a superconducting quantum interference device magnetometer; one sample has been prepared by oxygen intercalation, the other one by deintercalation. The susceptibility shows a peak near 45 K in both samples, which depends on the field and the magnetic history. The susceptibility of semiconducting and superconducting compounds, with an oxygen content O6.20 and O6.41, respectively, has also been investigated. The presence of the peak in the YBa2Cu3O6.30 samples has been explained by the combined effect of regions with a Curie-like behaviour and superconducting regions. The origin of these regions is discussed.